There have been growing demands in the process of semiconductor fabrication for transferring and receiving a wafer between various processing steps entirely in a clean space. Since semiconductor products tend to become easily defective when contaminated by impurities, they should preferably be conveyed in a clean room which is highly purified. As semiconductors are processed to smaller dimensions, the diameter of particles which cause problems is smaller. Therefore, it is difficult to produce desirable purified spaces in conventional clean rooms. It has been considered to convey a wafer or the like in a vacuum where particles are not suspended in the space because of less Brownian motion. Specifically, there has heretofore been proposed a magnetic levitation conveyor apparatus having a carriage for carrying a workpiece such as a wafer or the like in a vacuum maintained in a tunnel, the carriage being magnetically levitated for movement out of contact with a partition.
There has also been proposed a magnetic levitation conveyor apparatus having a carriage movable out of contact with a partition of a tunnel which is filled with a clean nitrogen gas or the like, rather than a vacuum.
For example, Japanese laid-open patent publications Nos. 61-295926 and 1-299119 disclose magnetic levitation vacuum conveyor apparatus in which a carriage that is magnetically levitated is movable along a conveyor passage in a partition maintained in a highly purified vacuum environment. Japanese laid-open patent publication No. 63-194502 and Japanese laid-open utility model publication No. 1-134998 disclose magnetic levitation vacuum conveyor apparatus of the type in which members that possibly produce dirt and dust particles are housed in the tunnel, i.e., levitating electromagnets, displacement sensors, and linear motors which are arrayed in a conveyor passage are housed in the tunnel (can), and a carriage is movable in a vacuum (purified atmosphere) outside of the tunnel (can).
Japanese laid-open patent publications Nos. 53-34272, 61-277303, 4-75404, and Japanese laid-open utility model publication No. 59-146326 disclose magnetic levitation conveyor apparatus having a carriage (movable body) suspended under magnetic attractive forces produced by levitating electromagnets arrayed along a conveyor passage for movement while being levitated off the conveyor passage.
In the magnetic levitation conveyor apparatus in which the carriage in a vacuum tunnel (partition) is movable while being suspended by levitating electromagnets disposed outside of the partition as disclosed in Japanese laid-open patent publication No. 61-295926, however, it is necessary to install a permanent magnet on the carriage, and the partition is required to be of a complex cross-sectional shape. Since various parts of the carriage and the partition itself discharge various gases and produce fine particles, it has been difficult to create a high degree of vacuum, and the workpiece carried by the carriage tends to be contaminated.
In the case where a purified space is produced by passing a nitrogen gas in a tunnel, it has been customary to place levitating electromagnets, linear motors, etc. in the tunnel. However, inasmuch particles deposited in gaps between electromagnet coils cannot fully be removed, these particles are discharged into the tunnel. Because the electromagnets are disposed in the tunnel, it has been quite laborsome to replace any of the electromagnet coils which is broken.
In the magnetic levitation vacuum conveyor apparatus of the type in which members that possibly produce dirt and dust particles are housed in the tunnel, cables for supplying a control current to the levitating electromagnets, the displacement sensors, or the linear motors in the tunnel (can) are exposed in a highly purified atmosphere such as a vacuum. Such an arrangement poses a problem on the handling of the cables, and the maintenance of components in the tunnel (can).
The present invention has been made in view of the above drawbacks. It is an object of the present invention to provide a magnetic levitation conveyor apparatus which has levitating electromagnets, linear motors, displacement sensors, etc. that are disposed outside of a tunnel, and a carriage movable in the tunnel, the carriage being of a simple structure, i.e., a canned structure, for preventing gases from being produced, the tunnel having a reduced cross section for being highly purified therein, and the carriage being well controllable to be levitated and moved in the tunnel.
The process of semiconductor fabrication employs a variety of processing apparatus for carrying out lithography, growing various films, and diffusing materials. These processing apparatus have to be interconnected by tunnels in which semiconductor wafers or the like need to be conveyed through a highly purified (high vacuum) atmosphere.
Accordingly, it is another object of the present invention to provide a practical magnetic levitation conveyor apparatus including a tunnel which has a branch, and a carriage that can be levitated in the tunnel and moved from a main conveyor passage into a branched conveyor passage at the branched point while being held out of contact with a partition of the tunnel.